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openssh/sandbox-seccomp-filter.c
Damien Miller 9e96b41682 Fix weakness in seccomp-bpf sandbox arg inspection 
Syscall arguments are passed via an array of 64-bit values in struct
seccomp_data, but we were only inspecting the bottom 32 bits and not
even those correctly for BE systems.

Fortunately, the only case argument inspection was used was in the
socketcall filtering so using this for sandbox escape seems
impossible.

ok dtucker
2017-03-14 12:41:53 +11:00

344 lines
8.8 KiB
C
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/*
* Copyright (c) 2012 Will Drewry <wad@dataspill.org>
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/*
* Uncomment the SANDBOX_SECCOMP_FILTER_DEBUG macro below to help diagnose
* filter breakage during development. *Do not* use this in production,
* as it relies on making library calls that are unsafe in signal context.
*
* Instead, live systems the auditctl(8) may be used to monitor failures.
* E.g.
* auditctl -a task,always -F uid=<privsep uid>
*/
/* #define SANDBOX_SECCOMP_FILTER_DEBUG 1 */
/* XXX it should be possible to do logging via the log socket safely */
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
/* Use the kernel headers in case of an older toolchain. */
# include <asm/siginfo.h>
# define __have_siginfo_t 1
# define __have_sigval_t 1
# define __have_sigevent_t 1
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
#include "includes.h"
#ifdef SANDBOX_SECCOMP_FILTER
#include <sys/types.h>
#include <sys/resource.h>
#include <sys/prctl.h>
#include <linux/net.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <elf.h>
#include <asm/unistd.h>
#include <errno.h>
#include <signal.h>
#include <stdarg.h>
#include <stddef.h> /* for offsetof */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include "log.h"
#include "ssh-sandbox.h"
#include "xmalloc.h"
/* Linux seccomp_filter sandbox */
#define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
/* Use a signal handler to emit violations when debugging */
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
# undef SECCOMP_FILTER_FAIL
# define SECCOMP_FILTER_FAIL SECCOMP_RET_TRAP
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
#if __BYTE_ORDER == __LITTLE_ENDIAN
# define ARG_LO_OFFSET 0
# define ARG_HI_OFFSET sizeof(uint32_t)
#elif __BYTE_ORDER == __BIG_ENDIAN
# define ARG_LO_OFFSET sizeof(uint32_t)
# define ARG_HI_OFFSET 0
#else
#error "Unknown endianness"
#endif
/* Simple helpers to avoid manual errors (but larger BPF programs). */
#define SC_DENY(_nr, _errno) \
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
#define SC_ALLOW(_nr) \
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
#define SC_ALLOW_ARG(_nr, _arg_nr, _arg_val) \
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 6), \
/* load and test first syscall argument, low word */ \
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
offsetof(struct seccomp_data, args[(_arg_nr)]) + ARG_LO_OFFSET), \
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, \
((_arg_val) & 0xFFFFFFFF), 0, 3), \
/* load and test first syscall argument, high word */ \
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
offsetof(struct seccomp_data, args[(_arg_nr)]) + ARG_HI_OFFSET), \
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, \
(((uint32_t)((uint64_t)(_arg_val) >> 32)) & 0xFFFFFFFF), 0, 1), \
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), \
/* reload syscall number; all rules expect it in accumulator */ \
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
offsetof(struct seccomp_data, nr))
/* Syscall filtering set for preauth. */
static const struct sock_filter preauth_insns[] = {
/* Ensure the syscall arch convention is as expected. */
BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
offsetof(struct seccomp_data, arch)),
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
/* Load the syscall number for checking. */
BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
offsetof(struct seccomp_data, nr)),
/* Syscalls to non-fatally deny */
#ifdef __NR_lstat
SC_DENY(lstat, EACCES),
#endif
#ifdef __NR_lstat64
SC_DENY(lstat64, EACCES),
#endif
#ifdef __NR_fstat
SC_DENY(fstat, EACCES),
#endif
#ifdef __NR_fstat64
SC_DENY(fstat64, EACCES),
#endif
#ifdef __NR_open
SC_DENY(open, EACCES),
#endif
#ifdef __NR_openat
SC_DENY(openat, EACCES),
#endif
#ifdef __NR_newfstatat
SC_DENY(newfstatat, EACCES),
#endif
#ifdef __NR_stat
SC_DENY(stat, EACCES),
#endif
#ifdef __NR_stat64
SC_DENY(stat64, EACCES),
#endif
/* Syscalls to permit */
#ifdef __NR_brk
SC_ALLOW(brk),
#endif
#ifdef __NR_clock_gettime
SC_ALLOW(clock_gettime),
#endif
#ifdef __NR_close
SC_ALLOW(close),
#endif
#ifdef __NR_exit
SC_ALLOW(exit),
#endif
#ifdef __NR_exit_group
SC_ALLOW(exit_group),
#endif
#ifdef __NR_getpgid
SC_ALLOW(getpgid),
#endif
#ifdef __NR_getpid
SC_ALLOW(getpid),
#endif
#ifdef __NR_getrandom
SC_ALLOW(getrandom),
#endif
#ifdef __NR_gettimeofday
SC_ALLOW(gettimeofday),
#endif
#ifdef __NR_madvise
SC_ALLOW(madvise),
#endif
#ifdef __NR_mmap
SC_ALLOW(mmap),
#endif
#ifdef __NR_mmap2
SC_ALLOW(mmap2),
#endif
#ifdef __NR_mremap
SC_ALLOW(mremap),
#endif
#ifdef __NR_munmap
SC_ALLOW(munmap),
#endif
#ifdef __NR__newselect
SC_ALLOW(_newselect),
#endif
#ifdef __NR_poll
SC_ALLOW(poll),
#endif
#ifdef __NR_pselect6
SC_ALLOW(pselect6),
#endif
#ifdef __NR_read
SC_ALLOW(read),
#endif
#ifdef __NR_rt_sigprocmask
SC_ALLOW(rt_sigprocmask),
#endif
#ifdef __NR_select
SC_ALLOW(select),
#endif
#ifdef __NR_shutdown
SC_ALLOW(shutdown),
#endif
#ifdef __NR_sigprocmask
SC_ALLOW(sigprocmask),
#endif
#ifdef __NR_time
SC_ALLOW(time),
#endif
#ifdef __NR_write
SC_ALLOW(write),
#endif
#ifdef __NR_socketcall
SC_ALLOW_ARG(socketcall, 0, SYS_SHUTDOWN),
#endif
/* Default deny */
BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
};
static const struct sock_fprog preauth_program = {
.len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
.filter = (struct sock_filter *)preauth_insns,
};
struct ssh_sandbox {
pid_t child_pid;
};
struct ssh_sandbox *
ssh_sandbox_init(struct monitor *monitor)
{
struct ssh_sandbox *box;
/*
* Strictly, we don't need to maintain any state here but we need
* to return non-NULL to satisfy the API.
*/
debug3("%s: preparing seccomp filter sandbox", __func__);
box = xcalloc(1, sizeof(*box));
box->child_pid = 0;
return box;
}
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
extern struct monitor *pmonitor;
void mm_log_handler(LogLevel level, const char *msg, void *ctx);
static void
ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
{
char msg[256];
snprintf(msg, sizeof(msg),
"%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
__func__, info->si_arch, info->si_syscall, info->si_call_addr);
mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
_exit(1);
}
static void
ssh_sandbox_child_debugging(void)
{
struct sigaction act;
sigset_t mask;
debug3("%s: installing SIGSYS handler", __func__);
memset(&act, 0, sizeof(act));
sigemptyset(&mask);
sigaddset(&mask, SIGSYS);
act.sa_sigaction = &ssh_sandbox_violation;
act.sa_flags = SA_SIGINFO;
if (sigaction(SIGSYS, &act, NULL) == -1)
fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
fatal("%s: sigprocmask(SIGSYS): %s",
__func__, strerror(errno));
}
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
void
ssh_sandbox_child(struct ssh_sandbox *box)
{
struct rlimit rl_zero;
int nnp_failed = 0;
/* Set rlimits for completeness if possible. */
rl_zero.rlim_cur = rl_zero.rlim_max = 0;
if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
__func__, strerror(errno));
if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
__func__, strerror(errno));
if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
__func__, strerror(errno));
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
ssh_sandbox_child_debugging();
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
__func__, strerror(errno));
nnp_failed = 1;
}
debug3("%s: attaching seccomp filter program", __func__);
if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
debug("%s: prctl(PR_SET_SECCOMP): %s",
__func__, strerror(errno));
else if (nnp_failed)
fatal("%s: SECCOMP_MODE_FILTER activated but "
"PR_SET_NO_NEW_PRIVS failed", __func__);
}
void
ssh_sandbox_parent_finish(struct ssh_sandbox *box)
{
free(box);
debug3("%s: finished", __func__);
}
void
ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
{
box->child_pid = child_pid;
}
#endif /* SANDBOX_SECCOMP_FILTER */
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